Geospace variability during the 2008–2009 Whole Heliosphere Intervals

Abstract

We simulate the ionosphere and thermosphere throughout the extended solar minimum epoch from 2008 to 2009 using geospace models, systematically validating the models with databases of observed geospace composition. We isolate and quantify observed changes of as much as 4 total electron content unit (TECU) (1 TECU = 10 16 elections m À2 ) (~36%) in global (60°SÀ60°N) ionospheric total electron density and as much as 19 × 10 À12 kg m À3 (~75%) in global thermospheric mass density at 250 km associated with fluctuating solar EUV radiation and geomagnetic activity during this nominally "quiet" period. Corresponding modeled responses to both solar EUV radiation and geomagnetic activity are about a factor of 2 smaller than is observed. We identify, as well, semiannual and annual oscillations that produce geospace variability comparable to that produced by external solar and geomagnetic influences, and which cause distinct differences among the three individual Whole Heliosphere Intervals.

Thermospheric mass density: A review

“…Using TIME-GCM, Qian et al (2014b) estimated that enhanced CO 2 cooling in 2008 compared to 1996 caused an interminimum density change of −5.8% at 400 km, in statistical agreement with the empirical estimate of −4.6%±1.9% by Emmert et al (2014a). Lean et al (2014) examined variations in thermospheric density over the course of the 2008 solar minimum (2008 and 2009). During this period, semiannual and annual oscillations were comparable in magnitude to the solar and geomagnetic activity influences, so that minimum overall density levels occurred in mid-2008 and mid-2009, even though the solar and geomagnetic forcings were at their respective minima at other epochs.…”

SAMI3/SD‐WACCM‐X simulations of ionospheric variability during northern winter 2009

“…One of the goals of our efforts to couple SAMI3 with an atmosphere model is to better capture the true variability in the ionosphere, which will ultimately lead to improvements in ionospheric forecasting. In previous work, we have performed simulations using the NRLSSI to specify the solar irradiance but used empirical models to specify the neutral winds and thermospheric composition [ Lean et al ., ; McDonald et al ., ]. Without meteorological weather, we were able to match variations such as the 27 day rotation of the Sun, the annual oscillation, and the semiannual oscillation.…”

“…The SAMI3 TEC in the EIA maxima is biased high (+18%) as compared with the JPL TEC (Figure ); this is not unexpected, given that previous comparisons of SAMI3 global mean TEC are systematically higher by ~20% [ Lean et al ., ] and are attributed to a thick topside. Weak daytime drifts contribute to the lack of well‐separated EIA crests, as compared with the JPL TEC.…”

On the reduced geoeffectiveness of solar cycle 24: A moderate storm perspective

“…As the Sun emerged from the deep solar minimum to the rising phase of the solar cycle 24, the sunspot number (SSN) was relatively small [ Gopalswamy et al ., ; Solomon et al ., ; Lean et al ., ; Potgieter et al ., ; Kilpua et al ., ]. Although SSN decreased by 40% in solar cycle 24, the CME rate was similar to that in cycle 23 [ Gopalswamy et al ., ].…”

“…CMEs cause severe storms while CIRs cause moderate storms [Gosling et al, 1991;Tsurutani and Gonzalez, 1997;Richardson et al, 2002;Tsurutani et al, 2006;Gopalswamy, 2008;Zhang et al, 2007]. The counterpart of CMEs in the interplanetary medium is termed as interplanetary coronal mass ejections (ICMEs), which are usually categorized as magnetic clouds (MCs) and nonmagnetic clouds or ejecta (EJ) based on their in situ plasma and magnetic signatures [Klein and Burlaga, 1982;Gopalswamy et al, 2010aGopalswamy et al, , 2010bRiley and Richardson, 2012, and references therein]. CIRs develop when high-speed solar wind streams (HSSs) emanating from coronal holes interact with streams of lower speed.…”